1. Field of the Invention
The present invention relates to release coatings for paper and related fibrous substrates. More specifically, it relates to coated paper which is generally classified in the art as release paper and which is characterized by its ability to separate intact from a surface which is normally adherent.
2. Description of the Prior Art
In a process for producing films or sheets of thermoplastic material, such as vinyl resins or polyurethane resins, the material in a flowable form, usually a plastisol, is deposited or "cast" on the supporting surface, heated to consolidate the thermoplastic material into a continuous self-supporting film, cooled and stripped from the support. A web of paper or equivalent flexible material having a layer of a "release coating" on its surface is typically used for supporting or carrying the film of thermoplastic material during the process of its consolidation. A release coating is one from which the cooled, consolidated film will readily part without being marred or distorted during separation.
When the web of flexible material is paper, the release coated paper web is generally referred to as "release paper". This release paper provides a surface against which the thermoplastic material forms and from which it will readily release and impart to the surface of the material the quality of finish of the release surface. When a textured surface is desired on the surface of the thermoplastic sheet material, the release paper is provided with an embossed surface, generally by mechanically embossing the paper prior to or following application of the release coating.
Release papers are also useful in applications other than for casting resinous films. For example, they are useful in the preparation of plastic laminates which are useful in various furniture articles such as cabinets, table tops and vanities, as well as paneling, wall coverings, partitions and doors. A conventional manner of forming these laminates is to place a lamination "sandwich" composed of a plastic layer and a desired substrate in a press and to mold them together by means of heat and pressure. The plastic layer may take the form of a resin impregnated or coated paper, known as a "prepreg." Alternatively, it may be in the form of a self-supporting plastic film. The substrate may be several fibrous layers, particle board or chip board.
When the laminate materials are placed in the press, a piece of release paper or other material having the release surface is placed in contact with the plastic layer on the side opposite the substrate. This not only prevents the plastic layer from adhering to the press, but, more importantly, imparts the surface characteristics of the release paper to the plastic surface to be molded. Thus, as a result of being molded in contact with a release coated surface, the plastic surface will have a smooth, textured, matte, glossy, or other desired finish. Obviously, because of the conditions in the press and the nature of the plastic, the release paper must have very good release characteristics. Examples of such plastic laminates and release coatings for such use are disclosed in U.S. Pat. No. 3,898,114 to Franklin J. Ward and U.S. Pat. No. 3,946,135 to John O. H. Peterson. These patents are herein incorporated by reference.
Another application where release papers are useful is for heat transferable printed designs, such as those used to heat transfer a design onto T-shirts, other articles of apparel and other suitable receptive surfaces. In this use, a design is printed upon a release paper for later transfer to these surfaces. The design may be printed with a polyvinyl chloride plastisol ink or it may be printed with an offset printing ink and overcoated with a layer of polyvinyl chloride plastisol. When placed against the receptive surface and heated the printed design and polyvinyl chloride plastisol overlayer, if one is used, transfers to the receptive surface where it sets upon cooling. In this application, the release surface must be capable of being printed upon and capable of releasing the printed material and overlying layer of polyvinyl chloride plastisol when it is transferred.
Usually, release papers comprise a base sheet provided with at least one first coating, conventionally referred to as a base coating, comprising a pigment or filler such as a paper coating grade clay together with an adhesive binder, and a second coating overlying the first and comprising an agent which provides the release function. The purpose of the base coating is to level and seal the surface. The pigment in the coating provides a smooth surface by masking the irregularities in the surface of the base sheet. The adhesive content of the coating is desired to prevent undue penetration of the subsequently applied release coating into the base sheet. In other words, the base coating provides a coating "hold up" function to keep the release agent at the surface while the coating dries. Any release agent which drains into the body of the base sheet would be ineffective to provide release. Therefore, to maximize the effectiveness of the amount of release agent applied, the base sheet is preferably base-coated to prevent penetration of the release coating into the body-stock. On the other hand, it is also desirable to reduce the number of coating operations performed. Therefore, it can be advantageous to provide the base coating and the release coating as a single coating.
Some release coatings are applied in a solvent system and others can be applied in an aqueous system. Solvent systems usually employ volatile materials which require special ventilation and safety precautions. Aqueous coatings have the advantage of a more convenient operation, but all release agents are not soluble in aqueous systems. Some release coatings are satisfactory for release from some materials, but not from others. For example, polyurethane films tend to release with difficulty from many release surfaces which are satisfactory for release of polyvinyl chloride.